Nickel-metal hydride rechargeable batteries are widely used as power sources for small electronic devices such as digital cameras and notebook personal computers due to their high energy density, and as power sources for electric automobiles and hybrid automobiles due to their suitability for high-power applications and excellent safety.
Such a nickel-metal hydride rechargeable battery normally includes a nickel electrode containing a positive active material mainly composed of nickel hydroxide, a negative electrode containing a hydrogen storing alloy as a main material, a separator, and an alkali electrolyte solution. Among these battery constituent materials, particularly the hydrogen storing alloy as a main material of the negative electrode significantly affects performance of the nickel-metal hydride rechargeable battery, such as a discharge capacity and an energy density. As the hydrogen storing alloy, various kinds of alloys have been heretofore examined.
Particularly, for the purpose of increasing the capacity of a nickel-metal hydride rechargeable battery, an attempt has been made to use a rare earth-Mg—Ni-based hydrogen storing alloy for a negative electrode (see Japanese Patent Laid-open Publication No. 11-323469).
However, the rare earth-Mg—Ni-based alloy has the problem that it is extremely poor in durability when used as a negative electrode of a battery. This is ascribable to occurrence of distortion among a plurality of crystal phases included in the alloy due to absorption and release of hydrogen associated with charge-discharge. There is also the problem that repetition of charge-discharge accelerates pulverization of the hydrogen storing alloy, leading to deterioration of durability. Further, the hydrogen storing alloy has the problem that it is easily corroded when the battery is stored under a high-temperature atmosphere, or charge-discharge is repeated.
In order to solve these problems, studies have been repeatedly conducted heretofore for optimizing the ratio of substitution elements or crystal phases (see, for example, Japanese Patent Laid-open Publication No. 2007-291474, Japanese Patent Laid-open Publication No. 2008-71684, Japanese Patent Laid-open Publication No. 2009-176712, Japanese Patent Laid-open Publication No. 2011-21262, Japanese Patent Laid-open Publication No. 2011-82129, Japanese Patent Laid-open Publication No. 2012-67357 and Japanese Patent Laid-open Publication No. 2014-114476), but an alloy having satisfactory corrosion resistance and durability has not been obtained yet.